In the field of gas turbine engines, much attention has been given in recent years, and much future effort will be directed toward, the concept generally referred to as "variable cycle engines." Though the exact definition of what particular characteristics are required of an engine to be classified as a variable cycle engine has not yet been established in the trade, some general characteristics are emerging. Generally, the term variable cycle engine encompasses a hybrid engine which can operate with characteristics approaching both the high bypass ratio dry (the term "dry" referring to non-afterburning and unaugmented) turbofan engine for optimum subsonic operation and the afterburning turbojet for optimum supersonic mission requirements.
In a gas turbofan engine a quantity of air is initially compressed by a fan disposed within an annular duct. A portion of this compressed air is normally ducted to a core engine where it is further compressed, combusted with a mixture of fuel, expanded through a turbine where energy is extracted to drive the compressor and fan, and discharged through a nozzle to provide a propulsive force. The remainder of the air compressed by the fan is ducted around the core engine where it too is exhausted through a nozzle to provide additional propulsive force. The ratio of the quantity of flow bypassing the core engine to that passing through it is referred to as "bypass ratio."
The need for variable cycle engines has arisen because of multimode or changing mission requirements of a particular engine. The need will become more severe in the future when more stringent federal aircraft noise standards are adopted since higher bypass ratio engines are inherently quieter than the same size engine (thrust-wise) of lower bypass ratio for reasons well known in the art. At the same time, such high bypass ratio engines are not necessarily the optimum configuration for supersonic aircraft performance. Current aircraft having multimission requirements are powered by engines which themselves are a compromise between a subsonic high bypass ratio turbofan and a supersonic augmented turbojet. As a result, what emerges is a compromised low-to-moderate bypass ratio turbofan engine.
General characteristics which are desirable in variable cycle gas turbine engines include:
1. Wide range in operating bypass ratio. In other words, high bypass ratio, low specific thrust operation for economical extended subsonic operation and low bypass ratio, high specific thrust operation for supersonic cruise. PA1 2. Ability to provide a given thrust over a considerable range of air flows to facilitate inlet air flow matching and to minimize installation drag. PA1 3. Ability to accomplish the aforementioned changes with a minimum variation in engine geometry and component aerodynamic loading. PA1 4. Ability to modulate between modes of operation smoothly and continuously. PA1 5. Mechanical and control simplicity.
The present invention incorporates a uniquely simple arrangement whereby all of the foregoing objectives can be accomplished.
The complexity of prior art approaches to the variable cycle engine has inhibited their development. Prior attempts have included retractable fans and inefficient combinations of variable area turbines and variable pitch fans and compressors. Some reported approaches involve a "compound" engine which is designed to utilize a pair of engines in tandem with one or both engines functioning depending upon the mode of operation. The weight associated with that portion of the engine which is unused during certain operating modes is an inherent disadvantage in aircraft applications.
A more recent attempt at designing a practical variable cycle engine involves inversion of fan streams flowing through concentric annular ducts (U.S. Pat. No. 3,792,584 -- Klees). The complexity of the necessary switch-over mechanism (rotation of one portion of the engine fan duct relative to the remainder) is one undesirable feature of such a concept, as is the increased length required for the system.
Other attempts involve switching a fan stream through alternative fan ducts. A valving mechanism is used to perform the switch over between the ducts. One undesirable feature of this type variable cycle engine is the fact that fan stall may be induced during the switching modes when flow may be retarded.
The present invention overcomes such disadvantages by providing for two separate fan ducts and means for modulating air flows therethrough without the necessity of "dead weight" redundancy or mechanical complexity.